Hydride compounds with silicon and germanium core atoms and method of synthesizing same

ABSTRACT

A method is provided for synthesizing silicon-germanium hydride compounds of the formula (H 3 Ge) 4-x SiH x , wherein x=0, 1, 2 or 3. The method includes combining a silane triflate with a compound having a GeH 3  ligand under conditions whereby the silicon-germanium hydride is formed. The compound having the GeH 3  ligand is selected from the group consisting of KGeH 3 , NaGeH 3  and MR 3 GeH 3 , wherein M is a Group IV element and R is an organic ligand. The silane triflate can be H x Si(OSO 2 CF 3 ) 4-x  or H x Si(OSO 2 C 4 F 9 ) 4-x . The method can be used to synthesize trisilane, (H 3 Si) 2 SiH 2 , and the iso-tetrasilane analog, (H 3 Si) 3 SiH, by combining a silane triflate with a compound comprising a SiH 3  ligand under conditions whereby the silicon hydride is formed. The silane triflate can include H x Si(OSO 2 CF 3 ) 4-x  or H x Si(OSO 2 C 4 F 9 ) 4-x  wherein x=1 or 2. A method for synthesizing (H 3 Ge) 2 SiH 2  includes combining H 3 GeSiH 2 (OSO 2 CF 3 ) with KGeH 3  under conditions whereby (H 3 Ge) 2 SiH 2  is formed.

RELATED APPLICATION DATA

This application is based on and claims the benefit of U.S. Provisional Patent Application No. 60/610,120 filed on Sep. 14, 2004, the disclosure of which is incorporated herein by this reference.

STATEMENT OF GOVERNMENT FUNDING

The United States Government provided financial assistance for this project through the National Science Foundation under Grant Nos. DMR-0221993, ARO DAAD19-00-1-0471 and AFRL/SNHC F19628-03-C-0056. Therefore, the United States Government may own certain rights to this invention.

BACKGROUND

This invention relates generally to silicon-germanium hydrides and silicon hydride analogs. More particularly, it relates to the synthesis of silicon-germanium hydrides having the molecular formula (H₃Ge)_(4-x)SiH_(x), wherein x=0, 1, 2 or 3 and silicon hydride analogs thereof.

Synthesis and development of electronic and optical materials as well as devices based on the Si—Ge and related group IV alloy semiconductor systems, such as Si—Ge—C and Si—Ge—Sn, is currently of interest due to the potentially useful electronic and optical properties of these systems. Commercial fabrication of such systems traditionally has been achieved via chemical vapor deposition (CVD) of disilane (SiH₃)₂ and digermane (GeH₃)₂. However, the development of new and useful materials based on these systems with device quality morphological and structural properties requires new low temperature growth methods. Trisilane, (H₃Si)₂SiH₂, is currently used for commercial growth of strained Si channel devices on Si—Ge buffered silicon. A major advantage of trisilane relative to traditional Si hydrides is its higher reactivity, allowing low temperature growth conditions compatible with development of strained Si channels. Previously reported methods for synthesizing trisilane, however, have significant drawbacks. They are based on electric silent discharge of lower Si-hydrides, which typically produce mixtures of materials at low yields. To isolate the trisilane product in pure form, complicated separation and purification procedures need to be employed.

Previous reports discuss the potential synthesis of tetragermylsilane, Si(GeH₃)₄. W. Dutton and M. Onyszchuk, Inorganic Chemistry volume 7, number 9, 1968. To the best of our knowledge, however, no definitive proof of its existence as a pure product possessing the correct stoichiometry has been provided thus far. The previously reported NMR data revealed a mixture of products and the reported elemental analysis corresponding to Si(GeH₃)₄ was incorrect. Furthermore, the synthetic method described is unsuitable for producing the compound for commercial applications such as for use as a CVD source.

L. Lobreyer and Sundermeyer Chem. Ber. 1991, 124(11), 2405-2410, have previously reported a synthetic method to the compound H₃Ge—SiH₃. Their synthetic methodology, however, did not afford high enough yields for practical use as a CVD source for the synthesis of semiconductor systems.

It an object of the present invention to provide compounds that display the necessary physical and chemical properties to be viable precursors for chemical vapor deposition (CVD) of Si—Ge semiconductors and related group IV alloys.

It is still another object of the present invention to provide a method for synthesizing such compounds that utilizes high-yield single-step substitution reactions involving commercially available starting materials.

It is yet another object of the present invention to provide a method for synthesizing trisilane that utilizes high-yield single-step substitution reactions involving commercially available starting materials.

Additional objects and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations pointed out in the appended claims.

SUMMARY

To achieve the foregoing objects, and in accordance with the purposes of the invention as embodied and broadly described in this document, there is provided a new and practical method for synthesizing the family of silicon-germanium hydride compounds of the molecular formula (H₃Ge)_(4-x)SiH_(x), wherein x is equal to 0, 1, 2 or 3. These compounds possess simple tetrahedral structures in which a single silicon center atom, Si, is bonded to a hydrogen atom, H, and one or more germyl groups, GeH₃. The compounds have facile reactivities and precise atomic arrangements incorporating direct Si—Ge bonds. We have prepared the (H₃Ge)₂SiH₂, (H₃Ge)₃SiH and (H₃Ge)₄Si species of this family of compounds for the first time and have isolated these species as volatile colorless liquids that display the necessary physical and chemical properties to be viable precursors for chemical vapor deposition (CVD) of Si—Ge semiconductors and related group IV alloys.

According to one aspect of the invention, the method includes combining a silane triflate with a compound comprising a GeH₃ ligand under conditions whereby the silicon-germanium hydride is formed. The compound comprising the GeH₃ ligand is selected from the group consisting of KGeH₃, NaGeH₃ and MR₃GeH₃, wherein M is a Group IV element and R is an organic ligand. According to one advantageous method, the silane triflate can comprise H_(x)Si(OSO₂CF₃)_(4-x) or H_(x)Si(OSO₂C₄F₉)_(4-x).

According to another aspect of the invention, an alternative method is provided for synthesizing (H₃Ge)₂SiH₂. The method comprises combining H₃GeSiH₂(OSO₂CF₃) with KGeH₃ under conditions whereby (H₃Ge)₂SiH₂ is formed.

According to yet another aspect of the invention, we have provided a new and practical method for synthesizing trisilane, (H₃Si)₂SiH₂. The same method can be used for synthesis of the iso-tetrasilane analog, (H₃Si)₃SiH. The method comprises combining a silane triflate with a compound comprising a SiH₃ ligand under conditions whereby the silicon hydride is formed. According to one advantageous method, the silane triflate comprises H_(x)Si(OSO₂CF₃)_(4-x) or H_(x)Si(OSO₂C₄F₉)_(4-x) wherein x=1 or 2.

We have synthesized trisilane in practical yields by a straightforward approach that is convenient and potentially less expensive than previously known methods, and we have thereby demonstrated that trisilane can be used as a low temperature single-source alternative to the commercially available derivatives such as disilane (SiH₃)₂ and digermane (GeH₃)₂ for industrial and research applications. Potentially, this method affords iso-tetrasilane (H₃Si)₃SiH in high yields. The more reactive iso-tetrasilane (H₃Si)₃SiH is expected to be a better candidate for low temperature CVD of strained Si layers. Our approach provides a rational and systematic step-by-step mechanism leading to the isolation of the desired materials as the primary product at high yield and purity excluding formation of hazardous byproducts and mixtures.

The synthetic routes of the aforementioned molecules utilize high-yield single-step substitution reactions involving commercially available starting materials. A complete characterization was conducted via a range of spectroscopic and analytical methods such as multinuclear NMR, gas source IR, mass spectrometry and elemental analysis for Si and Ge. The data collectively confirm the assigned molecular structures and correlate well with other related silyl and germyl silanes and methanes. The experimental results compare extremely well with first principles calculations of the spectroscopic and bonding properties of the molecules. A detailed investigation of the physical and chemical properties has shown that the compounds can be purified to yield semiconductor grade materials that are highly suitable for industrial application in Si-based technologies and manufacturing processes.

The synthesis method according to our invention presents a new and straightforward approach that can afford a low temperature CVD route to Ge rich Si_(1-x)Ge_(x) (x>50 at. %) alloys, which currently have crucial application in the areas of optoelectronic IR devices as well as buffer layers and virtual substrates with tunable morphology composition, structure and strain. These buffer layers are used as templates for growth of strained Si and Ge films (channels) that have applications in high mobility electronic devices.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate the presently preferred embodiments and methods of the invention. Together with the general description given above and the detailed description of the preferred embodiments and methods given below, they serve to explain the principles of the invention.

FIG. 1 shows the molecular structure of the most common configuration of SiH₃GeH₃ obtained according to the method of the present invention.

FIG. 2 shows the molecular structure of the most common configurations of SiGe₂H₈. The symmetric, (α) SiH₂(GeH₃)₂ has been obtained according to the method of the present invention.

FIG. 3 shows the molecular structure of the most common configuration of SiGe₃H₁₀. The symmetric, (α) SiH(GeH₃)₃ has been obtained according to the method of the present invention.

FIG. 4 shows the molecular structure of the most common configuration of Si(GeH₃)₄ obtained according to the method of the present invention.

FIG. 5 shows the normalized theoretical and experimental infrared spectra for SiH₂(GeH₃)₂ for (a) the low frequency portion of the spectrum and (b) the high frequency hydrogen bands.

FIG. 6 shows the normalized theoretical and experimental infrared spectra for SiH(GeH₃)₃ for (a) the low frequency portion of the spectrum and (b) the high frequency hydrogen bands.

FIG. 7 shows the normalized theoretical and experimental infrared spectra for Si(GeH₃)₄ for (a) the low frequency portion of the spectrum and (b) the high frequency hydrogen bands.

DESCRIPTION Synthesis of (H₃Ge)₂SiH₂, (H₃Ge)₃SiH and (H₃Ge)₄Si

According to one aspect of our invention, we provide a method for synthesizing compounds of digermylsilane (H₃Ge)₂SiH₂, trigermylsilane (H₃Ge)₃SiH and tetragermylsilane (H₃Ge)₄Si. A preferred method is carried out by reactions of the corresponding silane-triflates with appropriate concentrations of potassium germyl in suitable solvents as illustrated by the following equations: H_(x)Si(OSO₂CF₃)_(4-x)+4−xKGeH₃→(H₃Ge)_(4-x)SiH_(x)+4−xKOSO₂CF₃ (x=0-3) H_(x)Si(OSO₂C₄F₉)_(4-x)+4−xKGeH₃→(H₃Ge)_(4-x)SiH_(x)+4−xKOSO₂C₄F₉ (x=0-3) We have synthesized and fully characterized most of the triflate substrates utilized in our work for the first time. A relevant and useful silyl-triflate chemistry review was reported recently by W. Uhlig Chem. Ber. 1996, 129, 733, which is incorporated herein in its entirety by this reference. (a) Synthesis and Properties of Digermylsilane, (H₃Ge)₂SiH₂.

We have synthesized symmetric digermylsilane (H₃Ge)₂SiH₂ using the method described above. We conducted this synthesis by the reaction of H₂Si(OSO₂CF₃)₂, and potassium germyl, KGeH₃, as shown by the equation (1) below: H₂Si(OSO₂CF₃)₂+2KGeH₃→(H₃Ge)₂SiH₂+2KOSO₂CF₃  (1)

Reaction (1) is carried out in a high boiling point solvent, such as n-decane, at 0° C. The low vapor pressure of decane allows for a convenient and effective separation and purification of the compound from the solvent. The product is obtained in 20%-25% yield as a colorless, air-sensitive and volatile liquid with a vapor pressure of 30 torr at 22° C. and 17 torr at 0° C. H₃Ge—SiH₂—GeH₃ is thermally stable at 22° C., and it less reactive with air and much safer than the H₃GeGeH₃ analog. The substantial vapor pressure and sufficient thermal stability of H₃Ge—SiH₂—GeH₃ suggest that the molecule could be a highly suitable single-source CVD precursor to silicon-germanium semiconductor alloys. Notably, this compound appears to possess higher stability than the well known homonuclear analogs such as trisilane (H₃Si)₂SiH₂, and digermane H₃GeGeH₃ which are currently commercially available and are considered the gas sources of choice for low temperature deposition of Si-based devices including high mobility strained Si channels. H₃Ge—SiH₂—GeH₃ offers the possibility of becoming a safer and more efficient alternative to these compounds in the preparation of Si—Ge alloys with high Ge-rich concentrations. These alloys are much more difficult to grow in device quality form and are highly sought for important application in modern optical devices including ER photodetectors and sensors fully integrated with silicon technologies.

The (H₃Ge)₂SiH₂ compound is readily identified and characterized by its infrared (IR), NMR, and mass spectra. Its IR spectrum in vapor form is relatively simple and shows two sharp absorptions at 2152 cm⁻¹ and 2074 cm⁻¹ which are assigned to the Si—H and Ge—H stretching modes, respectively. These assignments are consistent with the literature values of the H₃SiGeH₃ compound as described by J. Urban, P. R. Schreiner, G. Vacek, P. v. R. Schleyer, J. Q. Huang, J. Leszczynski, Chem. Phys. Lett. 1997, 264, 441-448. The intensity of the Ge—H peak in the H₃Ge—SiH₂—GeH₃ spectrum is significantly stronger than the Si—H peak which is consistent with the greater number of Ge—H bonds versus Si—H bonds in the molecule. Other prominent absorptions at 805 cm⁻¹ and 702 cm⁻¹ are attributed to Si—H and Ge—H bending modes, respectively. A weak band at 324 cm⁻¹ can be attributed to the skeletal Si—Ge stretching mode. A more detailed interpretation of the full IR spectrum is obtained from ab initio quantum chemical calculations and it is described below. The mass spectrum of the compound displays well-defined isotopic envelopes for (M⁺-nH) and (M⁺-GeH₃ ⁺), suggesting a (H₃Ge)₂SiH₂ structure in which a central SiH₂ group is bonded with two terminal GeH₃ ligands. The ¹H NMR spectra are consistent with the proposed structure. The spectra show the expected triplet centered at 3.106 ppm (δ Ge—H) due to the GeH₃ moieties and a septet at 3.396 ppm (δ Si—H) due to SiH₂. The integrated Ge—H/Si—H proton ratio in the NMR spectrum is 3:1, as expected. The NMR frequencies also correlate well with the corresponding chemical shifts of SiH₃GeH₃, which are reported to be at 3.520 ppm and 3.180 ppm as Si—H and Ge—H quartets, respectively.

We have also prepared (H₃Ge)₂SiH₂ via an alternative two-step process. As shown below, we initially synthesized and isolated the monosubstituted H₃GeSiH₂(OSO₂CF₃) compound by reactions of equimolar amounts of PhSiH₂GeH₃ with HOSO₂CF₃ in the absence of solvent at −35° C. The synthesis of PhSiH₂GeH₃ is described in more detail by L. Lobreyer and Sundermeyer, Chem. Ber. 1991, 124(11), 2405-2410, which is incorporated herein in its entirety by this reference. Subsequent reactions of H₃GeSiH₂(OSO₂CF₃) with KGeH₃ affords the disubstituted (H₃Ge)₂SiH₂ product. PhSiH₂GeH₃+HOSO₂CF₃→H₃GeSiH₂(OSO₂CF₃)+C₆H₆  (2) H₃GeSiH₂(OSO₂CF₃)+KGeH₃→(H₃Ge)₂SiH₂+KOSO₂CF₃  (3)

We isolated the intermediate H₃GeSiH₂OSO₂CF₃ species for the first time as a colorless, low-volatility liquid (v.p.=8.0 Torr at 22° C., 3 Torr at 0° C.) at 72% yield. The neat product decomposes slowly at 22° C. over time to give SiH₃GeH₃ and an unidentified residue, but it remains stable when stored at −25° C. The compound was characterized by gas-phase IR, NMR (¹H, ¹³C, ¹⁹F) and mass spectrometry and the data collectively are consistent with the proposed H₃GeSiH₂OSO₂CF₃ molecular structure (see experimental section for details).

The synthesis of (H₃Ge)₂SiH₂, as described by reaction (2), gives a highly pure product in which the overall yield is slightly higher than obtained via the previous method. Nevertheless, the method shown in reaction (2) has afforded the formation of the new and highly reactive species, (H₃Ge)SiH₂(OSO₂CF₃), which might be a suitable starting material for the synthesis of other useful semiconductor specialty gases that incorporate the direct Si—Ge bonds.

Ab Initio Calculations of Digermylsilane, H₃Ge—SiH₂—GeH₃.

Our data indicate that we have isolated, for the first time, the Si—Ge hydride H₃Ge—SiH₂—GeH₃ with a symmetric propane-like molecular structure. Earlier work based on silent electric discharge of SiH₄ and GeH₄ produced the asymmetric analog H₃Ge—GeH₂—SiH₃, exclusively. See K. M. Mackay, S. T. Hosfield and S. R. Stobart, J. Chem. Soc. (A), 1969, 2938. This indicates that the symmetric species according to our invention might not be stable under the high-energy conditions of the silent discharge process.

To elucidate the structural, vibrational and energetic properties of H₃Ge—SiH₂—GeH₃ we conducted extensive electronic structure calculations which are based on hybrid density function theory (DFT) using the B3LYP functional as implemented in the Gaussian03 and GAMESS codes. A variety of basis sets were employed to study the structural and vibrational trends of H₃Ge—SiH₂—GeH₃ as well as the GeH₃—GeH₂—SiH₃ isomer. In addition the properties of the classical SiH₃GeH₃ analog were calculated for comparison (see Table 1). Earlier studies of SiH₃GeH₃ have established the importance of augmenting the basis sets by the inclusion of extra d-type polarization functions on heavy atoms (Si,Ge) and extra p-type polarization functions on the hydrogens. These studies are described by J. Urban, P. R. Schreiner, G. Vacek, P. v. R. Schleyer, J. Q. Huang, J. Leszczynski, Chem. Phys. Lett. 1997, 264, 441-448, which are incorporated herein in their entirety by this reference. Accordingly, we performed our calculations using 6-311++G(2d,2p) basis, which also includes additional diffuse orbitals on first and second row atoms.

FIG. 2 shows the structure of H₃Ge—SiH₂—GeH₃ obtained using the 6-311++G(2d,2p) basis set at the B3LYP level of theory. FIGS. 1, 3 and 4 show concomitant data for the SiH₃GeH₃, SiH(GeH₃)₃ and Si(GeH₃)₄ analogs, respectively, as well as those of plausible isomers. In FIGS. 1-4, hydrogen atoms are represented by white spheres, silicon atoms are represented by small dark gray spheres, and germanium atoms are represented by light gray spheres. Table 1 shows structural and energetic parameters of SiGeH₆ and the symmetric, (α), and asymmetric, (β), SiGe₂H₈ and SiGe₃H₁₀ molecules. E₀ and E_(th) refer to the static and thermally corrected (300° K) electronic molecular energies, respectively. Lengths are given in Angstroms, zero-point energies in kcal/mol, dipole moments in Debye and total energies in Hartree. For the asymmetric (β) species Ge₁ refers to central Ge atom in the structures of FIGS. 1-4.

Note that the data for the H₃Ge—SiH₂—GeH₃ and GeH₃—GeH₂—SiH₃ isomers are virtually identical. In particular the Si—Ge, Ge—H and Si—H bond lengths for the H₃Ge—SiH₂—GeH₃ are 2.397 Å, 1.539 Å, and 1.484 Å respectively. These results are slightly larger (+0.003 Å) than the calculated values for SiH₃GeH₃ which are in excellent agreement with the prior experimental and theoretical data reported by J. Urban, P. R. Schreiner, G. Vacek, P. v. R. Schleyer, J. Q. Huang, J. Leszczynski, Chem. Phys. Lett. 1997, 264, 441-448. The <Ge—Si—Ge bond angle (112.1°) is slightly larger than the tetrahedral value, but this is compensated by a slightly reduced value for the <H—Si—H angle (108°). Our calculations also show that the asymmetric isomer is more stable by ˜18 kcal/mol than its symmetric counterpart (see Table 1), which is consistent with the formation of the former almost exclusively in the silent discharge experiments.

TABLE 1 Structural and Energetic Parameters Symmetric(α) Asymmetric (β) Symmetric (α) Asymmetric (β) iGeH₆ SiH₂(GeH₃)₂ (SiH₃)GeH₂(GeH₃) SiH(GeH₃)₃ SiGe₃H₁₀ Si—Ge 2.394 Si—Ge 2.397 Si—Ge₁ 2.398 Si—Ge 2.401 Si—Ge₂ 2.401 Ge₁—Ge₂ 2.445 Ge₁—Ge₂ 2.450 Ge—H 1.539 Ge—H 1.539 Ge₁—H 1.543 Ge—H 1.539 Ge₂—H 1.546 Ge₂—H 1.539 Ge₁—H 1.539 Si—H 1.481 Si—H 1.484 Si—H 1.482 Si—H 1.487 Si—H 1.482 <GeSiGe 112.1 <SiGe₁Ge₂ 113.1 <GeSiGe 110.6 <SiGe₂Ge₁ 113.1 <H_(Si)SiGe 108.4 <H_(Ge)SiGe 108.4 <H_(Ge)GeSi 110.5 <H_(Ge)GeSi 110.9 <HSIH 108.0 <HGe₁H 107.1 <H_(Ge)GeH_(Ge) 107.3 <H_(Ge)GeH_(Ge) 110.6 <H_(Si)SiH_(Si) 110.6 ZPE 29.8 ZPE 39.2 ZPE 39.9 ZPE 49.5 ZPE 49.8 D 0.323 D 0.445 D 0.147 D 0.731 D 0.761 E₀ + 2370 −0.1351 E₀ + 4448 −0.3574 E₀ + 4448 −0.3603 E₀ + 6526 −0.5753 E₀ + 6526 −0.5868 E_(th) + 2370 −0.1156 E_(th) + 4448 −0.3240 E_(th) + 4448 −0.3303 E_(th) + 6526 −0.5358 E_(th) + 6526 −0.5463

The IR spectrum for H₃Ge—SiH₂—GeH₃ was also calculated using the B3LYP functional and the 6-311++G(2d,2p) basis set. FIG. 5 compares the calculated and experimental IR spectra and Table 2 summarizes the frequencies of key vibrational bands and their corresponding assignments and compares the observed and calculated values. Gratifying agreement between experiment and theory is obtained using a uniform frequency scale factor of 0.989 for the low frequency bands (˜<1000 cm-1). For the high frequency Si—H and Ge—H vibrations a scale factor of 0.98 is found to yield optimal correspondence with experiment. The latter value was also obtained by Urban et al. in their treatment of Si₂H₆, Ge₂H₆ and SiH₃GeH₃.

TABLE 2 Vibrational mode assignments for the symmetric SiH₂(GeH₃)₂ molecule Ob- Calcu- served lated ν(cm⁻¹) ν(cm⁻¹) SiH₂(GeH₃)₂ Description Low Frequency Vibrational Bands (λ = 0.989) — 313.8 1 Symmetric H—Si, H—Ge ∥ wagging 423.8 406.8 2 Asymmetric H—Si, H—Ge ∥ wagging 510.7 502.2 3 Symmetric Ge—H ∥ wagging 540.0 547.8 4 Asymmetric H—Si, H—Ge ⊥ wagging 639.6 — 651.8 — 696.1 704.1 5 Symmetric Si—H ∥ wagging + asymmetric Ge—Si stretching 725.1 — 771.1 — 796.5 805.4 6 Symmetric H—Ge wagging + H—Si ∥ wagging 803.9 812.6  6′ Symmetric H—Ge wagging 882.3 883.9 7 Asymmetric H—Ge ⊥ wagging 888.8 884.0  7′ Symmetric H—Ge ⊥ wagging 917.0 — 926.5 932.6 8 Strong symmetric H—Si—H ⊥ bending High Frequency Vibrational Bands (λ = 0.98) 2013.7 — 2066.0 2053.7 1 Symmetric H—Ge stretching (anti- symmetric w.r.t. Ge sites) 2074.1 2067.8 2 Symmetric H—Ge stretching 2071.1 Asymmetric H—Ge stretching (symmetric w.r.t. Ge sites) 2109.7 — 2140.7 2151.5 3 Symmetrical H—Si stretching 2152.8 2163.5 4 Asymmetrical H—Si stretching 2163.6 —

We also calculated the IR spectrum of the asymmetric GeH₃—GeH₂—SiH₃ isomer (not shown) and found that it is in excellent agreement with the data reported previously by K. M. Mackay, S. T. Hosfield and S. R. Stobart, J. Chem. Soc. (A), 1969, 2938. A comparison between the IR spectra of the GeH₃—SiH₂—GeH₃ (FIG. 5) and GeH₃—GeH₂—SiH₃ corroborates the NMR findings that in our experiments we produced almost exclusively the symmetric (GeH₃)₂SiH₂ analog.

Example Synthesis of Digermylsilane, H₃Ge—SiH₂—GeH₃

The following example helps to further explain the invention. It will be understood, however, that the example is illustrative of the invention and that the invention is not limited only to this example.

H₃Ge—SiH₂—GeH₃. A liquid sample of H₂Si(OTf)₂ (4.38 g, 13.3 mmol) was added dropwise via an addition funnel to a slurry of solid KGeH₃ (4.0 g, 34.9 mmol) in 40 mL of dry decane. The slurry was prepared in a 250 mL, two-neck flask using a 33% excess of KGeH₃. The addition funnel was attached to the flask and the reaction assembly was evacuated to 0.200 torr. The H₂Si(OTf)₂ was added at 0° C. The resulting mixture was stirred at ambient temperature for 2 hours after which it was distilled for several hours under dynamic vacuum through a series of −45° C. and −196° C. traps to collect the volatiles. Gas phase IR revealed that the −45° C. trap contained a small fraction of decane and the −196° C. trap contained primarily (H₃Ge)₂SiH₂ with traces of H₃SiGeH₃, SiH₄ and GeH₄. The contents of the −196° C. trap were fractionally distilled through a series of U-traps held at −45° C., −90° C., −130° C. and −196° C. Pure (H₃Ge)₂SiH₂ and H₃SiGeH₃ were collected at −90° C. and −130° C., respectively, while SiH₄, and GeH₄ were obtained in the −196° C. trap. Traces of solvent were found in the −45° C. trap and subsequently discarded.

From this process, pure digermylsilane (H₃Ge)₂SiH₂ was obtained in a 20-25% yield further characterized as follows:

Vapor pressure: 30 Torr (22° C.), 17 Torr (0° C.).

IR (gas, cm⁻¹): 2152 (m), 2074 (vs), 2010 (w, sh), 927 (w), 917 (w), 883 (w), 803 (s), 769 (vw), 723 (vw), 696 (vs), 649 (vw), 641 (vw), 547 (vw), 515 (vw), 424 (vw), 330 (vw).

¹H-NMR (CDCl₃, 7.24): δ 3.395 (septet, Si—H₂, coupling constant 4 Hz), δ 3.110 (triplet, Ge—H₃, coupling constant 4 Hz) Use ³J_((HH))=4 Hz, ¹J_((SiH))=200 Hz, etc.

²⁹Si-NMR: δ=−102.45.

EIMS (m/e): isotopic envelopes centered at 174 (M⁺-nH), 148 (Ge₂H_(x) ⁺), 106 (H₃SiGeH₃ ⁺), 75 (GeH₄ ⁺), 31 (SiH₄ ⁺).

H₃GeSiH₂(OSO₂CF₃).

Trifluoromethane sulfonic acid, HOSO₂CF₃, (0.645 g, 4.3 mmol) was added dropwise to pure PhSiH₂GeH₃ (0.784 g, 4.3 mmol) at −35° C. under nitrogen. The formation of frozen benzene was immediately observed during the addition process. After addition the mixture was stirred for 30 minutes at −35° C. and subsequently distilled through a series of U-traps held at −25° C. and −196° C. under dynamic vacuum. Gas-phase IR revealed the presence of H₃GeSiH₂(OSO₂CF₃) in the −25° C. trap and a mixture of H₃SiGeH₃ and benzene in the −196° C. trap. The identity and purity of the H₃GeSiH₂(OSO₂CF₃) was further established by mass spectrometry and NMR spectroscopy. The Si—H and Ge—H stretching modes were observed at 2155 cm⁻¹ and 2071 cm⁻¹, respectively, indicating the presence of the SiH₂GeH₃ moiety. A series of bands between 1450 cm⁻¹ and 1100 cm⁻¹ revealed the presence of the triflate (OSO₂CF₃) group. The ¹H NMR resonance revealed a quartet at 5.430 ppm and a triplet at 3.514 ppm corresponding to silyl and germyl proton resonances consistent with the SiH₂GeH₃ group. The ¹⁹F and ¹³C-NMR spectra revealed the presence of a single fluorine chemical shift at −76.34 ppm and a ¹³C quartet at 114-122 ppm, which is consistent with the CF₃ moiety of the triflate group.

From this process, 0.760 g of H₃GeSiH₂(OSO₂CF₃) was obtained as a colorless, low volatility liquid in a 72% yield further characterized as follows:

Vapor pressure: 8.0 Torr at 22° C., 3 Torr at 0° C.

IR (gas, cm⁻¹): 2194 (w, sh), 2155 (s), 2071 (vs), 1425 (m), 1253 (w), 1225 (ms), 1158 (m), 1077 (s), 964 (ms), 951 (ms), 883 (s), 863 (ms), 844 (w), 792 (s), 763 (vs), 690 (vw), 625 (w), 467 (vw), 434 (vw), 355 (vw).

¹H-NMR (CDCl₃): δ 5.430 (quartet, Si—H, coupling constant 3.3 Hz), δ 3.514 (triplet, Ge—H, coupling constant 3.3 Hz).

¹³C-NMR (CDCl₃): δ 122-114 (quartet, CF₃). ¹⁹F-NMR (CDCl₃, δ): −76.34 (s, CF₃).

EIMS (m/e): isotopic envelopes centered at 179 (M⁺-GeH₃), 149 (CF₃SO₃ ⁺), 150 (Ge₂H_(6-x) ⁺), 106 (H₃SiGeH₃ ⁺), 75 (GeH₄ ⁺), 69 (CF₃ ⁺), 32 (SiH₄ ⁺).

(b) Synthesis and Properties of Trigermylsilane SiH(GeH₃)₃

We have also synthesized trigermylsilane (H₃Ge)₃SiH using a similar procedure to that employed for the (H₃Ge)₂SiH₂ analog described above. The preparation was conducted by the reaction of HSi(OSO₂CF₃)₃, and a slight excess of KGeH₃ as shown by the equation below. HSi(OSO₂CF₃)₃+3KGeH₃→(H₃Ge)₃SiH+3KOSO₂CF₃  (4)

The product was obtained in ˜30% yields as a colorless, air-sensitive and volatile liquid with a vapor pressure (of 6-7 Torr at 22° C.) and is stable at 22° C. Extensive NMR, mass spectrometric and IR (including first principle simulations) characterizations indicated that the molecular structure comprised a central Si atom that is bonded to three GeH₃ and one H groups similar to isobutane. The ¹H NMR spectra showed a deciplet at 3.429 ppm and a doublet at 3.317 ppm for the Si—H and Ge—H proton signals, respectively. The splitting patterns of the Si—H and Ge—H protons and their corresponding integrated peak ratio of 1:9 is consistent with the isobutane-like (H₃Ge)₃SiH structure. Furthermore, a ²⁹Si-HMQC spectrum revealed a chemical shift of −112.73 ppm and that the ¹H-NMR signal at 3.429 ppm is coupled with the silicon in trigermylsilane. A ¹H-COSY experiment confirmed that the Si—H and Ge—H chemical shifts are coupled with one another as expected. The mass spectra show an isotopic envelop at 255-238 amu as the highest mass peak corresponding SiGe₃H_(x). Although the molecule appears to lose readily H in the mass spectrometer the SiGe3 core is retained in the gas phase. The IR spectrum of the compound shows the characteristic Si—H and Ge—H stretching modes at 2071 cm⁻¹ and 2132 cm⁻¹, respectively, and a set of absorptions at 881 cm⁻¹ 788 cm⁻¹, and 680 cm⁻¹ correspond to prominent bending modes of the molecule. The IR spectrum for (H₃Ge)₃SiH was also calculated using the B3LYP functional and the 6-311++G(2d,2p) basis set. FIG. 6 compares the calculated and experimental IR spectra and Table 3 summarizes the frequencies of key vibrational bands and their corresponding assignments and compares the observed and calculated values. A close agreement between experiment and theory is obtained.

TABLE 3 Vibrational mode assignments for the symmetric SiH(GeH₃)₃ molecule. Ob- Calcu- served lated ν(cm⁻¹) ν(cm⁻¹) SiH(GeH₃)₃ Description Low Frequency Vibrational Bands (λ = 0.989) 407.2 401.2 1 Symmetric H—Si, H—Ge ∥ wagging 512.3 511.9 2 Symmetric GeH₃ group rocking 613.4 — 644.0 — 680.5 685.7 3 H—Si wagging 745.6 — 787.1 787.2 4 H—Si wagging + H—Ge counter- wagging 820.2 821.4 5 Symmetric, concerted radial wagging of all H—Ge's 882.3 880.0 6 Asymmetric (w.r.t. Ge) radial wagging 880.8 of all H—Ge's 883.8 929.4 — High Frequency Vibrational Bands (λ = 0.98) 2020.3 — 2059.8 2066.4 1 Asymmetric H—Ge stretching (anti- 2068.0 symmetric w.r.t. Ge sites) 2072.1 2080.8 2 Asymmetric H—Ge stretching (⊥ to H—Si bond) 2082.9 Symmetric H—Ge stretching (⊥ to H—Si bond) 2084.3 Symmetric H—Ge stretches (∥ to H—Si bond) 2132.2 2143.2 3 H—Si stretching 2140.1 2157.6 —

Example Synthesis of Trigermylsilane, (H₃Ge)₃SiH

The following example helps to further explain the invention. It will be understood, however, that the example is illustrative of the invention and that the invention is not limited only to this example.

A liquid sample of HSi(OTf)₃ (4.10 g, 8.6 mmol) was added drop wise via an addition funnel to a slurry of solid KGeH₃ (4.5 g, 39.2 mmol) in 40 mL of dry ether at −35° C. The slurry was prepared in a 100 mL, two-neck flask using a 20% excess of KGeH₃. The addition funnel was attached to the flask and the reaction assembly was evacuated to 300 torr. The resulting mixture was stirred at −35° C. for 30 minutes after which it was slowly warmed to ambient temperature over the course of 90 minutes. A colorless solid was observed and the mixture was stirred at ambient temperature for 5 hrs. The volatiles were distilled into a U-trap held at −196° C. under dynamic vacuum for 2½ hours. The contents of the trap were redistilled through a series of traps held at −40° C. (H₃Ge)₃SiH and trace ether), −78° C. (ether and traces of H₃Ge)₂SiH₂) and 196° C. (ether and traces of GeH4). The product (−40° C. trap) was obtained by repeated distillation through −25° C. and −78° C. traps with no pumping. Gas phase IR revealed that the −25° C. trap contained trigermylsilane and the −78° C. trap contained a small amount of (H₃Ge)₂SiH₂.

From this process, trigermylsilane (H₃Ge)₃SiH was obtained in a yield of 25-30% further characterized as follows:

Vapor pressure: 6 torr (22° C.).

IR (gas, cm⁻¹): 2132 (m), 2071 (vs), 2010 (w, sh), 922 (vw), 881 (w), 788 (vs), 745 (vw), 680 (s), 634 (vw), 606 (w), 514 (vw), 409 (vw).

¹H-NMR (CDCl₃, 7.24, at 5° C.): δ 3.429 (deciplet, Si—H, coupling constant 4 Hz), δ 3.317 (doublet, Ge—H₃, coupling constant 4 Hz):

²⁹Si-NMR (CDCl₃, at 5° C.): δ −112.73.

GCMS: m/z 255-238 (SiGe₃H_(x)), 230-213 (Ge₃H_(x)), 185-170 (SiGe₂H_(x))_(,) 154-140 (Ge₂H_(x)), 109-100 (SiGeH_(x)), 77-71 (GeH_(x)).

(c) Synthesis and Properties of Tetragermylsilane Si(GeH₃)₄

The preparation of the tetragermylsilane (H₃Ge)₄Si was conducted by the reaction of Cl₂Si(OSO₂CF₃)₂, ClSi(OSO₂CF₃)₃ or Si(OSO₂CF₃)₄ with KGeH₃ as shown by the following equations: Cl₂Si(OSO₂CF₃)₂+4KGeH₃→(H₃Ge)₃Si+2KOSO₂CF₃+2KCl  (5) ClSi(OSO₂CF₃)₃+4KGeH₃→(H₃Ge)₃Si+3KOSO₂CF₃+KCl  (6) Si(OSO₂CF₃)₄+4KGeH₃→(H₃Ge)₄Si+4KOSO₂CF₃  (7) The (H₃Ge)₄Si compound is a colorless liquid with ˜1-2 Torr vapor pressure at 22° C. and was characterized by FTIR, NMR and GCMS. The high symmetry of the molecule leads to an extremely simple IR spectrum, which shows absorptions at 2072 cm⁻¹ and 2062 cm⁻¹ corresponding to the symmetric and asymmetric Ge—H stretches respectively. The peak positions and relative intensities in the MK closely match closely the calculated spectrum (see FIG. 7 and Table 4 for theoretical and experimental IR spectra and corresponding peak assignments). No Si—H vibrational modes are detected by MR. The NMR spectrum shows a singlet at 3.40 ppm, which confirms the presence of single-environment GeH₃ ligands in the molecule. The highest mass peak in the GCMS spectrum is observed in the range of 328-318 amu. The peak position and the isotopic distribution indicate SiGe4H_(x) type species consistent with the Si(GeH₃)₄ tetrahedral structure of the molecule. The spectroscopic data collectively provide strong evidence for the successful synthesis and isolation of Si(GeH₃)₄. Moreover, a vapor pressure of ˜1-2 Torr for the compound is within the expected value and is similar to that reported for C(GeH₃)₄ (1-2 torr).

TABLE 4 Vibrational mode assignments for the Si(GeH₃)₄ molecule Observed Calculated ν(cm⁻¹) ν(cm⁻¹) Si(GeH₃)₄ Description Low Frequency Vibrational Bands (λ = 0.989) 778.2 781.6 1 Symmetric & Asymmetric axial H—Ge bending (rigid SiGe4 framework) 880.6 886.8 2 Asymmetrical H—Ge wagging High Frequency Vibrational Bands (λ = 0.98) 2002.5 — 2019.0 — 2060.2 2051.1 3 Symmetric H—Ge stretching (anti- symmetric w.r.t. Ge sites) 2070.8 2066.3 4 Symmetric H—Ge stretching 2134.0 —

Example Synthesis of Tetragermylsilane, (H₃Ge)₄Si

The following example helps to further explain the invention. It will be understood, however, that the example is illustrative of the invention and that the invention is not limited only to this example.

A 3-neck, 100 mL round bottom flask was charged with 2.98 g (26 mmol) of KGeH₃ and 50 mL of dry diethyl ether to form a solution. A liquid sample of Si(OTf)₄ (2.40 g, 6 mmol) was subsequently added to KGeH₃/ether over the course of 30 minutes at −50° C. The pressure in the reaction assembly was reduced to 300 torr prior to the addition of Si(OTf)₄. Once the addition was completed, the flask was slowly warmed to ambient temperature over the course of 2-3 hours. A build up of a colorless solid was observed in the reaction pot and the mixture was stirred at ambient temperature for 90 minutes after which the volatiles were condensed into a U-trap at −196° C. under dynamic vacuum. The contents of the −196° C. trap were redistilled through a −45° C. trap to collect a mixture of (H₃Ge)₃SiH and (H₃Ge)₄Si as colorless liquids. Final fractionation of the mixture was accomplished by distillation through U-traps cooled at −10° C. and −45° C. Gas-phase IR spectroscopy revealed that the −10° C. trap contained (H₃Ge)₄Si and the −45° C. trap contained (H₃Ge)₃SiH.

From this process, tetragermylsilane, (H₃Ge)₄Si, was obtained, characterized as follows:

Vapor pressure: 1-2 Torr (22° C.).

IR (gas, cm⁻¹): 2130 (vw, sh), 2072 (s), 2062 (s), 2020 (vw, sh), 881 (vw), 777 (vs), 680 (vw), 632 (vw).

¹H-NMR (C₆D₆, 7.15): δ 3.400 (singlet, Ge—H).

GCMS: m/z 328-318 (SiGe4Hx), 302-292 (Ge4Hx), 255-238 (SiGe3Hx), 230-213 (Ge3Hx), 185-170 (SiGe2Hx), 154-140 (Ge2Hx), 109-100 (SiGeHx), 77-71 (GeHx).

Our method has provided pure (H₃Ge)₄Si product in 5-10% yield. As evidenced by mass spectrometry (GCMS), gas phase IR, NMR data, our method produces a compound with the correct stoichiometry and with the correct ¼ Si:Ge elemental ratio in the compound. Our spectroscopic data was further corroborated by ab initio theoretical studies.

(d) Methods of Synthesis and Properties of Trisilane, H₂Si(SiH₃)₂, and iso-tetrasilane, HSi(SiH₃)₃

The previously described triflate-based methods to prepare silyl-germanes were also employed to synthesize the homologous trisilane, (H₃Si)₂SiH₂, as described by the equation: H₂Si(OSO₂CF₃)₂+2KSiH₃→(H₃Si)₂SiH₂+2KOSO₂CF₃  (8) As will be apparent to one of skill in the art, the iso-tetrasilane (H₃Si)₃SiH analogs can be obtained as described by the equation: HSi(OSO₂CF₃)₃+3KSiH₃→(H₃Si)₃SiH+3KOSO₂CF₃  (9)

Trisilane is currently the preferred gas source for commercial CVD growth of strained Si channel devices with highly enhanced electronic properties such as high electron and hole mobilities. The strained films are formed on Si substrates via graded buffer layers of Si—Ge using low temperature growth conditions to prevent strain relaxation and formation of defects. A major advantage of trisilane in strained Si applications relative to traditional Si hydrides such as SiH₄ and Si₂H₆ is its higher reactivity, leading to facile dehydrogenations at the required low-temperature range.

The tetrasilane (H₃Si)₃SiH compound has not yet been used in CVD of Si-based devices, although it is expected to be more suitable for low-temperature growth applications because of its higher reactivity and hence lower decomposition temperature relative to trisilane. The branched structure suggests that this compound is likely to be stable and possess significant volatility at room-temperature comparable to trisilane. It is thus expected to be a viable CVD source for commercial, large-scale applications and its full development warrants immediate consideration. There are several accounts for the possible existence of this compound but no definitive synthesis route and characterization of its properties have been reported to date.

Example Synthesis of Trisilane, Si₃H₈

The following example helps to further explain the invention. It will be understood, however, that the example is illustrative of the invention and that the invention is not limited only to this example.

A 3-neck, 100 mL round bottom flask was charged with 1.60 g (22.8 mmol) of KSiH₃ and 40 mL of ether was subsequently added to form a light-orange solution. A liquid sample of H₂Si(OTf)₂ (3.58 g, 10.9 mmol) was subsequently added to KSiH₃/ether over the course of 30 minutes at −40° C. The pressure in the reaction assembly was reduced to 10 torr prior to the addition of H₂Si(OTf)₂. Frothing was immediately observed upon addition of the triflate in which the volatiles were frequently trapped during the addition process by passing through −78° C. and −196° C. traps. Once the addition was completed, the flask was slowly warmed to ambient temperature with frequent trapping. A build up of an off-white solid was observed in the reaction pot and the mixture was stirred at ambient temperature for 30 minutes. The volatiles from each trap were subsequently combined and redistilled through −125° C. and −196° C. U-traps with no pumping. The contents of each trap were checked by gas-phase FTIR spectroscopy, which revealed the presence of trisilane (−125° C.) and a mixture of silane and disilane (−196° C.).

This process yielded 80 mg of trisilane, Si₃H₈, was obtained in a yield of 15%-20% and further characterized as follows:

IR (gas, cm⁻¹): 2210 (vw, sh), 2161 (vs), 1003 (vw, br), 941 (m), 880 (vs), 802 (vw), 748 (vw), 716 (s), 705 (s), 673 (vw), 589 (vw, br), 472 (vw), 442 (vw).

EIMS (m/z): 92-84 (Si₃H_(8-x)), 62-56 (Si₂H_(6-x)), 32-28 (SiH_(4-x)).

CONCLUSION

The above-described invention possesses numerous advantages as described herein. The invention in its broader aspects is not limited to the specific details, representative devices, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the general inventive concept. 

What is claimed is:
 1. A method of synthesizing a silicon hydride (H₃Si)_(4-x)SiH_(x), wherein x is equal to 1 or 2, the method comprising: combining a silane triflate with a compound comprising a SiH₃ ligand under conditions whereby the silicon hydride is formed, wherein the silane triflate comprises a compound selected from the group consisting of H_(x)Si (OSO₂CF₃)_(4-x) and H_(x)Si(OSO₂C₄F₉)_(4-x).
 2. The method of claim 1 wherein the silane triflate comprises H_(x)Si(OSO₂CF₃)_(4-x).
 3. The method of claim 1 wherein the silane triflate comprises H_(x)Si(OSO₂C₄F₉)_(4-x).
 4. A method of synthesizing a compound (H₃Si)₂SiH₂, the method comprising: combining H₂Si (triflate)₂, with KSiH₃ under conditions whereby (H₃Si)₂SiH₂ is obtained according to the equation: H₂Si (triflate)₂+2KSiH₃→(H₃Si)₂SiH₂+2 K(triflate).
 5. The method of claim 4 wherein the triflate comprises OSO₂CF₃.
 6. The method of claim 4 wherein the triflate comprises OSO₂C₄F₉.
 7. A method of synthesizing a compound (H₃Si)₃SiH, the method comprising: combining HSi (triflate)₃ with KSiH₃ under conditions whereby(H₃Si)₃SiH is obtained according to the equation: HSi (triflate)₃+3KSiH₃→(H₃Si)₃SiH+3 K(triflate).
 8. The method of claim 7 wherein the triflate comprises OSO₂CF₃.
 9. The method of claim 7 wherein the triflate comprises OSO₂C₄F₉. 